1. Field of the Invention
This invention relates to a pneumatic tire, particularly a radial tire for passenger car capable of reducing the weight without sacrificing the steering stability, wear resistance and the like.
2. Description of Related Art
As a method of reducing a weight of a pneumatic radial tire comprising, for example, a radial carcass comprised of one or more carcass plies and a belt disposed on an outer periphery side of a crown portion of the radial carcass and comprised of one or more belt layers, there are proposed the thinning of rubber gauge in the tire, the construction of the belt with aramid fiber cords, the simplification of the belt structure itself and the like.
When the aramid fiber cord is used as a cord of the belt layer or the belt structure is simplified by decreasing the number of belt layers, however, there are caused such problems in the basic performances of the tire that the rigidity of the tread portion lowers and hence the high steering stability, wear resistance and the like can not be ensured as compared with the case of adopting the conventionally general belt structure using steel cord as a cord of the belt layer or extending cords of the belt layers so as to cross the cords of the layers with each other. This is substantially true even in the case that the rubber gauge in each constructional part of the tire is decreased to lower the rigidity of the tire casing.
It is, therefore, an object of the invention to solve the above problems of the conventional technique and to provide a pneumatic tire capable of effectively reducing the tire weight without lowering the steering stability, wear resistance and the like.
According to the invention, there is the provision of in a pneumatic tire comprising a carcass comprised of one or more carcass plies and a belt disposed on an outer periphery side of a crown portion of the carcass and comprised of one or more belt layers, the improvement wherein each of at least one carcass ply and at least one belt layer is constituted with organic fiber cords, and the organic fiber cord in the carcass ply has a modulus of elasticity of 9.8-24 kN/mm2 and the organic fiber cord in the belt layer has a modulus of elasticity of 24-40 kN/mm2.
In the tire according to the invention, at least one of the belt layers is constituted with the organic fiber cords, e.g. aramid fiber cords, so that the belt weight and hence tire weight can effectively be reduced as compared with the case that all of the belt layers are constructed with steel cords. In this case, it is obliged to lower the rigidity of the tread portion as mentioned in connection with the conventional technique. According to the invention, however, at least one of the carcass plies is constituted with the organic fiber cords having a modulus of elasticity higher than that of the conventional cord to increase the rigidity of the tire casing to thereby compensate for the lowering of the rigidity in the tread portion, whereby the degradation of the steering stability, wear resistance and the like due to the lowering of the rigidity in the tread portion can sufficiently be prevented. On the other hand, the decrease of the rubber gauge can be allowed without lowering the rigidity of the casing.
When the modulus of elasticity of the organic fiber cord as a carcass ply cord is less than 9.8 kN/mm2, the rigidity of the tire casing can not be increased as is expected, while when it exceeds 24 kN/mm2, the rigidity of the tire casing becomes too high and the ride comfort onto the vehicle is degraded, and further separation failure is apt to be caused at cord ends of the carcass ply turned around the bead core and hence there is a fear of degrading the durability of the tire.
On the other hand, when the modulus of elasticity of the organic fiber cord as a belt layer cord is less than 24 kN/mm2, the rigidity of the tread portion is too low and hence such a rigidity lacking can not be compensated even by the increase of the rigidity of the casing as mentioned above, while when it exceeds 40 kN/mm2, the peeling of the belt layer cord from coating rubber therefor and premature fatigue of the cord due to compression input are caused and hence there is feared the degradation of the durability due to cord breakage in the belt layer or the like.
In a preferable embodiment of the invention, the cords in the belt layer are obliquely extended at a cord angle of 20-50xc2x0 with respect to an equatorial plane of the tire, whereby a large cornering power is developed and also an excellent wear resistance can be realized. Moreover, when the cord inclination angle exceeds 50xc2x0, there is a fear of degrading the durability in high-speed running.
In the tire according to the invention, it is favorable that at least one belt reinforcing layer containing cords such as nylon fiber cords, polyethylene naphthalate cords, aramid fiber cords and the like is arranged at an outer periphery side of the belt so as to extend subatantially in a circumferential direction of the tire.
In this case, the pushing-out deformation due to the action of centrifugal force or the like is effectively restrained under the function inherent to the belt reinforcing layer, while the belt reinforcing layer is contributed to increase the rigidity of the tread portion, whereby the steering stability, wear resistance and the like can be more advantageously improved in addition to the durability in high-speed running.
It is favorable that the modulus of elasticity of the cord in the belt reinforcing layer is made not less than 3.0 kN/mm2 for more effectively developing the above effect. For example, therefore, when the belt is comprised of the single belt layer, steel cord having a higher modulus of elasticity can be selected in addition to the above various organic fiber cords as a cord of the belt reinforcing layer. Thus, the weight reduction of the tire is ensured as compared with the conventional technique, and all of the durability in high-speed running, steering stability, wear resistance and the like can be more improved.
When the modulus of elasticity of the cord in the belt reinforcing layer is less than 3.0 kN/mm2, the belt reinforcing layer can not largely be contributed to the improvement of the durability in high-speed running and the increase of the rigidity of the tread portion.